This invention relates to the field of electronic controllers for controlling a switched reluctance machine.
Electric motors turn electrical energy into mechanical energy to produce work. Electric motors work by applying a voltage across one or more inductors, typically a machine winding, thereby energizing the inductor(s) to produce a resultant magnetic field. Mechanical forces of attraction or repulsion caused by the magnetic field cause a rotor in an electric motor to move. The efficiency of the electrical motor depends in part on the timing and magnitude of each application of voltage to the motor. Timing of the voltage being applied is particularly important in the case of switched reluctance motors.
Historically, the switched reluctance motor was thought to be incapable of competing effectively with other types of motors. More recently, however, a better understanding of motor design and the application of electronically controlled switching has resulted in a robust switched reluctance drive capable of high levels of performance over a wide range of sizes, powers and speeds. Note that the term xe2x80x98motorxe2x80x99 is used here, but it will be appreciated by those skilled in the art that the term covers the same machine in a generating mode unless a particular distinction is made.
The switched reluctance motor is generally constructed without conductive windings or permanent magnets on the rotating part (called the rotor) and includes electronically-switched windings carrying unidirectional currents on the stationary part (called the stator). Commonly, pairs of diametrically opposed stator poles may be connected in series or parallel to form one phase of a potentially multi-phase switched reluctance motor. Motoring torque is developed by applying voltage to each of the phase windings in a predetermined sequence that is synchronized with the angular position of the rotor so that a magnetic force of attraction results between poles of the rotor and stator as they approach each other. Similarly, generating action is produced by positioning the voltage pulse in the part of the cycle where the poles are moving away from each other.
The general theory of design and operation of switched reluctance motors is well known and discussed, for example, in The Characteristics Design and Applications of Switched Reluctance Motors and Drives, by Stephenson and Blake and presented at the PCIM ""93 Conference and Exhibition at Nuremberg, Germany, Jun. 21-24, 1993.
The control of the torque of a switched reluctance machine (and hence speed and rotor position) is made possible by the positioning and wave-shaping of the excitation currents in the motor windings with respect to the rotor positions. In certain known switched reluctance machine systems, the positioning and shaping of the excitation currents is controlled through the use of peak current values and turn-ON and turn-OFF angles where: (i) a peak current value represents a desired peak current level over a given interval of rotor rotation; (ii) a turn-ON angle defines a point during the rotor""s rotation when voltage is applied to the phase winding (referred to as the xe2x80x9cturn-ON anglexe2x80x9d); and (iii) a turn-OFF angle defines a point during the rotor""s rotation when the application of voltage to the winding is halted. Various control schemes (e.g., pulse width modulation, bang-bang current chopping and others) may be used to control the voltage that is applied to a phase winding during the intervals defined by the turn-ON and turn-OFF angles for that phase winding.
The relationships of the control inputs for a switched reluctance machine system (e.g., turn-on angles, turn-off angles, current levels, current wave-shapes, etc.) with the outputs (speed, torque, rotor position, etc.) are complicated, and a full utilization of all the control inputs to maximize performance represents a non-linear many-to-many mapping problem that is difficult for conventional control design. As such, in known switched reluctance motor systems, this complex relationship is typically implemented through the use of a relatively complex circuit that stores signals representative of the relationship between the speed, torque demand, and turn-ON and turn-OFF angles of the motor. Circuits of this type are commonly referred to as xe2x80x9ccontrol law tables.xe2x80x9d
In many known controllers for switched reluctance motors, the control law table comprises a circuit that includes turn-ON and turn-OFF angle information for various rotor speed and torque demand combinations. In most systems, the information that is stored in the control law table is derived empirically through a process commonly known as xe2x80x9ccharacterizationxe2x80x9d in which the appropriate turn-ON and turn-OFF angles required to produce the torque demand are determined for a number of different rotor speeds. The empirically derived information is then stored in the control-law table, sometimes together with information from non-tested speeds and torque demands which has been interpolated from the empirically derived information.
One disadvantage of conventional controllers that use control law tables is the necessity of providing for the control law circuitry. For a switched reluctance machine system that works in a wide speed range and a wide torque range and for stability purpose, the control law table must be densely populated, which means that a large data memory is needed. Moreover, because each control law table is the result of the characterization of a particular switched reluctance machine, a control law based controller is a motor-specific controller and cannot be properly used with other switched reluctance machines having designs and sizes different from the machine used to generate the data in the control law table. As such, control law based controllers adds to the cost and time required to develop a new control system, in that, the process of characterizing the machine used in the system may be required for each new motor and controller.
The present invention provides a circuit and a method for controlling a switched reluctance motor without the use of large, costly control law tables. The present invention also provides a compact and generalized controller and control technique for intelligent control of a switched reluctance machine that satisfies desired control requirements and that has lower development and construction costs and better performance than conventional switched reluctance controllers and control techniques.
Aspects of the present invention provide a compact and generalized technique for intelligent control of a switched reluctance machine that can be easily implemented in a low cost digital signal processor or microcontroller. Such aspects of the present invention allow for the construction of a low-cost high-performance controller.
In accordance with one embodiment of the present invention, a controller for a switched reluctance machine is provided that includes a fuzzy-logic control circuit that: (i) receives as an input a signal representing the difference between a desired operating characteristic of the switched reluctance machine and the actual operating characteristic of the switched reluctance machine; and (ii) provides at its output an intermediate control signal corresponding to the desired output torque of the reluctance machine; and (iii) a multi-layer neural network circuit that receives at its input the intermediate control signal from the fuzzy-logic control circuit and a signal representing the rotational speed of the switched reluctance machine and provides at its output control signals for controlling energization of the switched reluctance machine.
In accordance with another embodiment of the present invention, a controller for a switched reluctance machine is provided that includes a proportional-integral control circuit that receives as input a speed error signal representing the difference between the desired operating speed of the switched reluctance machine and the operating speed of the switched reluctance machine, the proportional-integral controller providing output signals that is the sum of a component proportional to the input and a component proportional to the integral of the input; and a multi-layer neural network circuit that receives at one input the outputs from the proportional-integral control circuit and at another input a signal representing the rotational speed of the switched reluctance machine, the neural network providing at its output control signals for controlling energization of the switched reluctance machine.
In accordance with yet another embodiment of the present invention, a fuzzy logic control circuit for controlling the energization of a switched reluctance machine is provided where the control circuit includes fuzzification means for receiving a signal corresponding to the difference between a desired output characteristic of the switched reluctance machine and the actual output characteristic of the machine and for generating a fuzzy set representing the input signal; a knowledge-based fuzzy inference engine that receives at its inputs the fuzzy set generated by the fuzzification means, the inference-engine generating a fuzzy output control set utilizing a rule-base that corresponds to the general operating characteristics of switched reluctance machines; and de-fuzzification means for receiving the output of the fuzzy inference engine and providing output control signals for controlling the energization of the switched reluctance machine.
Other exemplary embodiments of the present invention and other features of the present invention will be apparent to one of ordinary skill in the art having the benefit of this disclosure.